The present invention relates generally to techniques for improving vehicle fuel efficiency, and more particularly, to improving the fuel efficiency of unoccupied autonomous vehicles by operating at more aggressive effective reduced engine displacement levels, without concern for occupant noise, vibration and harshness (NVH) considerations.
Most vehicles in operation today (and many other devices) are powered by internal combustion (IC) engines. Internal combustion engines typically have a plurality of cylinders or other working chambers where combustion occurs. Under normal driving conditions, the torque generated by an internal combustion engine needs to vary over a wide range in order to meet the operational demands of the driver. The fuel efficiency of many types of internal combustion engines can be substantially improved by varying the displacement of the engine. This allows for the full torque to be available when required, yet can significantly reduce pumping losses and improve fuel efficiency through the use of a smaller displacement when full torque is not required. The most common method of varying the displacement today is deactivating a group of cylinders substantially simultaneously. In this approach, no fuel is delivered to the deactivated cylinders and their associated intake and exhaust valves are kept closed as long as the cylinders remain deactivated.
Another engine control approach that varies the effective displacement of an engine is referred to as “skip fire” engine control. In general, skip fire engine control contemplates selectively skipping the firing of certain cylinders during selected firing opportunities. Thus, a particular cylinder may be fired during one engine cycle and then may be skipped during the next engine cycle and then selectively skipped or fired during the next. Skip fire engine operation is distinguished from conventional variable displacement engine control in which a designated set of cylinders are deactivated substantially simultaneously and remain deactivated as long as the engine remains in the same displacement mode. Thus, the sequence of specific cylinder firings will always be exactly the same for each engine cycle during operation in any particular variable displacement mode (so long as the engine maintains the same displacement), whereas that is often not the case during skip fire operation. For example, an 8-cylinder variable displacement engine may deactivate half of the cylinders (i.e. 4 cylinders) so that it is operating using only the remaining 4 cylinders. Commercially available variable displacement engines available today typically support only two or at most three fixed displacement modes.
In general, skip fire engine operation facilitates finer control of the effective engine displacement than is possible using a conventional variable displacement approach. For example, firing every third cylinder in a 4-cylinder engine would provide an effective displacement of ⅓rd of the full engine displacement, which is a fractional displacement that is not obtainable by simply deactivating a set of cylinders. Conceptually, virtually any effective displacement can be obtained using skip fire control, although in practice most implementations restrict operation to a set of available firing fractions, sequences or patterns. The Applicants, has filed a number of patents describing various approaches to skip fire control. By way of example, U.S. Pat. Nos. 7,849,835; 7,886,715; 7,954,474; 8,099,224; 8,131,445; 8,131,447; 8,464,690; 8,616,181; 8,839,766; 8,869,773; 9,086,020; 9,120,478; 9,175,613; 9,200,575; 9,291,106; 9,399,964 and others, describe a variety of engine controllers that make it practical to operate a wide variety of internal combustion engines in a skip fire operational mode. Each of these patents is incorporated herein by reference.
The Applicant has filed a number of patents describing various approaches to skip fire control. By way of example, U.S. Pat. Nos. 8,099,224; 8,464,690; 8,651,091; 8,839,766; 8,869,773; 9,020,735; 9,086,020; 9,120,478; 9,175,613; 9,200,575; 9,200,587; 9,291,106; 9,399,964, and others describe a variety of engine controllers that make it practical to operate a wide variety of internal combustion engines in a dynamic skip fire operational mode. Each of these patents is incorporated herein by reference. Many of these patents relate to dynamic skip fire control in which firing decisions regarding whether to skip or fire a particular cylinder during a particular working cycle are made in real time—often just briefly before the working cycle begins and often on an individual cylinder firing opportunity by firing opportunity basis.
In some applications referred to as dynamic multi-level skip fire, individual working cycles that are fired may be purposely operated at different cylinder outputs levels—that is, using purposefully different air charge and corresponding fueling levels. By way of example, U.S. Pat. No. 9,399,964 describes some such approaches. The individual cylinder control concepts used in dynamic skip fire can also be applied to dynamic multi-charge level engine operation in which all cylinders are fired, but individual working cycles are purposely operated at different cylinder output levels. Dynamic skip fire and dynamic multi-charge level engine operation may collectively be considered different types of dynamic firing level modulation engine operation in which the output of each working cycle (e.g., skip/fire, high/low, skip/high/low, etc.) is dynamically determined during operation of the engine, typically on an individual cylinder working cycle by working cycle (firing opportunity by firing opportunity) basis. It should be appreciated that dynamic firing level modulation engine operation is different than conventional variable displacement in which when the engine enters a reduced displacement operational state a defined set of cylinders are operated in generally the same manner until the engine transitions to a different operational state.
Recently there have been extensive efforts to develop vehicles with the ability to drive autonomously. Although fully autonomous vehicles are not commercially available today, enormous progress has been made in their development and it is expected that self-driving vehicles will become a commercial reality in the relatively near future. When viable, autonomous vehicles will have the capability to operate without intervention by a human driver, that is, an on-board autonomous driving unit will have the ability to make decisions about acceleration, braking, stopping, steering, etc.
Since autonomous vehicles will have the ability to safely operate without human intervention, there are a number of scenarios where vehicles may operate unoccupied. For instance, a person may “summon” an unoccupied vehicle to pick them up at a designated location and drive them to another location. Or, a vehicle operating as a shuttle or taxi may autonomously drive unoccupied between the passenger drop-offs and pick-ups. In yet other examples, vehicles may operate “unmanned” in a variety of situations, such as delivery trucks, construction vehicles, farming vehicles, operation of vehicles in hazardous or dangerous areas, etc.
In an occupied vehicle, one of the constraints with any of the above-listed variable displacement methods is that, overly rough operation of the engine and/or drive train may result at certain engine displacements. As a consequence, occupants may experience an unacceptable level of noise, vibration and harshness, often referred to in the automotive industry as “NVH”. Thus, with any of the above-listed variable displacement methods, the displacement level may be intentionally constrained to maintain acceptable levels of NVH, but at the expense of further fuel savings.
The Applicant has realized that with unoccupied autonomous vehicles, NVH constraints to preserve occupant comfort are no longer an issue. The present application is therefore directed to a wide variety of fuel-saving variable engine displacement strategies for unoccupied autonomous vehicles, under a host of different operating conditions, for reducing fuel consumption without NVH concerns for vehicle occupants.